Was the imagery of the flower female sex organ alluding to yurika performing oral on aoyama or the other way around?
It reminds me of Alien in that the aliens use an unconsenting party for reproduction (sex, which is associated with close relationships). They force a parasytic relationship which can be an allegory for toxic and abusive relationships. Yurika did the same but this form of reproduction is meant to produce another link in a chain of abuse rather than literal reproduction to produce offspring. Many perpetrators were victims of abuse as a child.
What I'm horrified to presume is that yurika really took it that far. Not just forcing her into a dubious emotional/social relationship. It really seems to imply she did take it that far. Literally forcing it into aoyama's mouth the same way the aliens do in Alien. Yurika belongs in prison if this is the case. Even moreso now that it seems she came back, potentially for the purpose of further tormenting her victim, and probably to check whether aoyama continued the "tradition" of an older girl raping a younger one.
Yurika was a victim before becoming a perpetrator. It's a good thing aoyama didn't want to take the same path as a means of closure. Whether it was because no fish bit the hook or because aoyama hasn't been able to free herself from having feelings towards her abuser is something to ponder. Nonetheless, yurika's return is incredibly concerning. Whether she came back because she wants to apoligize for her crime or did so to encourage the continuation of the abuse remains to be seen.
One last thing to note is whether yurika read all of aoyama's messages because she couldn't completely let aoyama go. She could have blocked aoyama but didn't. She could've left the message unread but didn't. And now she came back in a role that is directly connected to this whole situation. She's a literature teacher. Class s left such an impression/trauma on her that even in adulthood she can't let it go. She came back to the scene of the crime. A crimescene where she was both the victim and (later) the perpetrator. Returning to the place where it all started means she couldn't let it go. Their link in the cycle of abuse hasn't solidified yet. Meaning, it isn't set in stone that the cycle will continue. Our protags are the figurative beacons of hope for the end of it.
(I was too tired to check for spelling mistakes. I don't use auto correct.)